I know someone who uses one.
It's quite amazing, eg. globulars. Nebulae spectacular with a Ha filter.
a) Glass won't become obsolete as it still requires an eyepiece for magnification.
b) it's very expensive for high quality.... prohibitively (like, the cost of a top-tier telescope). And yuo still need the scope (and the eyepieces)
c) also, you don't get colour in NV